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Review Low-Carbon Sustainable Composites from Waste Phosphogypsum and Their Environmental Impacts
Kai Ren 1,2, Na Cui 3,*, Shuyuan Zhao 4, Kai Zheng 2, Xia Ji 2, Lichao Feng 1, Xin Cheng 2 and Ning Xie 2,4
1 School of Mechanical Engineering, Jiangsu Ocean University, Lianyungang 222005, China; [email protected] (K.R.); [email protected] (L.F.) 2 Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China; [email protected] (K.Z.); [email protected] (X.J.); [email protected] (X.C.); [email protected] (N.X.) 3 School of Civil Engineering and Architecture, University of Jinan, Jinan 250022, China 4 National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, China; [email protected] * Correspondence: [email protected]; Tel.: +86-15105311232
Abstract: Phosphogypsum (PG) is an industrial waste from the production of phosphoric acid and phosphate fertilizer. Disposal and landfill of PG pose significant environmental problems due to its hazardous components. Although many researchers have explored the possibility of PG recycling, challenges still exist before it can be high-effectively reused. In particular, a great deal of recent attention has been attracted to explore using PG as raw material to manufacture sustainable Citation: Ren, K.; Cui, N.; Zhao, S.; composites. The impurities movement, recycling efficiency, and environmental impacts have to be Zheng, K.; Ji, X.; Feng, L.; Cheng, X.; further investigated. This review article summarized the state of the art of the purification process, Xie, N. Low-Carbon Sustainable application areas, and the environmental impacts of PG waste. The main challenges and potential Composites from Waste application approaches were discussed. This article is focused on reviewing the details of the PG Phosphogypsum and Their reusing which benefits the readers on learning the knowledge from previous efforts. The main Environmental Impacts. Crystals 2021, challenges of reusing PG were discussed from the chemical, physical, and materials perspectives. 11, 719. https://doi.org/10.3390/ cryst11070719 Keywords: phosphogypsum; building materials; composites; recycling; environmental impact
Academic Editors: Francesco Capitelli, Iwona Skoczko, Edyta Pawluczuk and Enrique Fernandez Ledesma 1. Introduction PG is a by-product of industrial phosphoric acid production. For every ton of phos- Received: 13 May 2021 phoric acid produced, 4–5 tons of PG will be produced [1,2]. At present, the output of PG is Accepted: 15 June 2021 increasing rapidly at the rate of about 70 million tons per year. In China, the total amount Published: 22 June 2021 of PG is about 500 million tons, but the utilization rate is less than 15% [3,4]. The general treatment method of PG is to accumulate as waste, which was piled up and thus posing Publisher’s Note: MDPI stays neutral severe environmental problems [5,6]. Due to the distinctive dark side of the PG disposal, with regard to jurisdictional claims in the research on recycling utilization of the PG as resources to manufacture useful materials published maps and institutional affil- has become an ever-burgeoning demand. iations. 2. Purification
The essence of PG and natural gypsum is the calcium sulfate dihydrate (CaSO4·2H2O). The major difference between the natural gypsum and phosphogypsum is that PG contains Copyright: © 2021 by the authors. various impurities which result in the performance of PG was far inferior to the natural Licensee MDPI, Basel, Switzerland. gypsum [7]. The impurities in phosphogypsum include soluble phosphate, fluorides, or- This article is an open access article ganic matters, and many radioactive elements such as Ra226, Pb210, Po210,U238, and U234 [8]. distributed under the terms and Although it was reported that the concentration of these impurities is categorized as a safe conditions of the Creative Commons practice according to the standards of the Environmental Protection Agency (EPA) [9], the Attribution (CC BY) license (https:// purification of the PG is still compelling because the impurities have significant impacts creativecommons.org/licenses/by/ on the setting time, mechanical properties, and durability of the PG products [10]. Table1 4.0/).
Crystals 2021, 11, 719. https://doi.org/10.3390/cryst11070719 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 719 2 of 20
lists the impurity composition and their existing forms in PG [11–16]. Provided with appropriate treatment methods, PG can replace natural gypsum in many scenarios, which provides a possibility for large-scale and industrial utilization of PG. As a result, it is critical to remove the impurities from the PG before it can be widely applied in diverse fields. The impurities and their forms in PG are listed in Table1. To eliminate the impurities from the PG, the most widely used approaches include chemical methods, physical methods, and heat treatment methods [17–19].
Table 1. Impurities in PG and their existing forms.
Impurity Type Solubility Main Existing Forms Effects Corrosive to equipment, 3− 2− Phosphate PO4 , HPO4 , prolonging the setting time Soluble 4− [7,15,20] H2PO ,H3PO4 of building gypsum and weakening its strength. Prolonging setting time and Eutectic CaHPO ·2H O weakening the strength of morphology 4 2 gypsum hardened body. Phosphate complexes (with Al, Fe, alkali Insoluble The impact was small. metals, etc.), decomposed apatite Promote coagulation, Reduce Fluoride soluble F− the strength of the gypsum.
Insoluble Na3AlF6, CaSiF6, CaF The impact was small. Plant roots mixed in phosphate rock, The water requirement of Defoamer, scale standard consistency Organic matter Insoluble remover, and crystal increases. The porosity of the transformation agent hardened body increases and added in the the strength of PG decreases. production process Result in powdering and Other impurities soluble K+, Na+ frosting on the surface of PG [21] products. Quartz, Oxides of Sr, The effect of oxide was not Fe, and Mg or sulfate palpable, radioactive Insoluble complexes of Sr and elements do great harm to Fe, Ra226, Pb210, organisms. Po210,U238, and U234
2.1. Chemical Methods Acids are the most widely used chemical that can be used as an agent to purify the PG and improve its quality that ensures its applications in other places. For example, it was found that using an aqueous citric acid solution is an effective approach to eliminate phosphoric and fluoride impurities [20]. The impurities removing process was facile. By shaking the mixture of PG and 2–5% citric acid solution for 25 min at 30 ◦C, the mixture was filtered and washed with plain water three times followed by drying at 42 ◦C. In this process, the phosphates and fluoride compounds in PG will react with citric acid to form water-soluble phosphoric acid (H3PO4), sodium citrate (Na3(C6H8O7), hydrofluoric acid (HF), hydrofluorosilicic acid (H3SiF6), hydro-fluoro aluminate (H3AlF6) and hydro-fluoro ferrate (H3FeF6). These water-soluble chemicals can be removed by the water stream, and realize the purification purpose. Sulfuric acid is another aqueous solution that can be used as an agent to purify the PG. In this method, an aqueous sulfuric acid solution with a concentration of 5% was mixed Crystals 2021, 11, x FOR PEER REVIEW 3 of 20
form water-soluble phosphoric acid (H3PO4), sodium citrate (Na3(C6H8O7), hydrofluoric acid (HF), hydrofluorosilicic acid (H3SiF6), hydro-fluoro aluminate (H3AlF6) and hy- dro-fluoro ferrate (H3FeF6). These water-soluble chemicals can be removed by the water Crystals 2021, 11, 719 stream, and realize the purification purpose. 3 of 20 Sulfuric acid is another aqueous solution that can be used as an agent to purify the PG. In this method, an aqueous sulfuric acid solution with a concentration of 5% was mixed with PG powder and stirred with a speed of 150 rpm for 30 min at 30 °C and with PG powder and stirred with a speed of 150 rpm for 30 min at 30 ◦C and 60 ◦C. After 60 °C. After floating for 30–45 min, the pulp was filtered, washed by water, and followed floating for 30–45 min, the pulp was filtered, washed by water, and followed by drying at by drying at 65 °C for 24 h (the purification process is shown in Figure 1). The tempera- 65 ◦C for 24 h (the purification process is shown in Figure1). The temperature, volume of ture, volume of the aqueous sulfuric acid solution, PG quantity, contents of P/F/orgainic the aqueous sulfuric acid solution, PG quantity, contents of P/F/orgainic carbon, and pH carbon, and pH value were selected as variables to evaluate the purification effects. The value were selected as variables to evaluate the purification effects. The results indicate results indicate that the optimized parameters should be 3 L sulfuric acid solution mixed that the optimized parameters should be 3 L sulfuric acid solution mixed with 1 kg PG with 1 kg PG and stirred at 60 °C. The ICP testing result has proven that the concentra- and stirred at 60 ◦C. The ICP testing result has proven that the concentration of impurities tion of impurities has been remarkably reduced and the purified gypsum plaster can be has been remarkably reduced and the purified gypsum plaster can be used as a building materialused as a [ 22building]. material [22].
Figure 1. Flow chart of the purificationpurification process of PG with thethe sulfuricsulfuric acidacid solutionsolution [[22].22].
In Inaddition addition to tovarious various acids, acids, the the PG PGcan can be purified be purified by ammonium by ammonium hydroxide. hydroxide. In this In thisprocess, process, PG was PG wasmixed mixed with with aqueous aqueous ammonium ammonium hydroxide hydroxide with with the concentration the concentration var- ◦ variedied from from 5% 5% to to20%. 20%. After After shaking shaking for for 24 24 h hat at 35 35 °C,C, the the samples samples were were filtered, filtered, washedwashed with 0.5% ammoniumammonium hydroxide solution followedfollowed by water. The samples were finallyfinally ◦ dried atat 4242 °CC toto obtainobtain the the purified purified gypsum. gypsum The. The solubility solubility of of the the phosphatic phosphatic and and fluoride fluo- compoundsride compounds increased increased with with increasing increasing the concentration the concentration of ammonium of ammonium hydroxide. hydroxide. The monocalciumThe monocalcium phosphate phosphate and sodiumand sodium fluoride fluoride are more are solublemore soluble than other than compounds.other com- Comparingpounds. Comparing with the mineralwith the gypsum mineral and gypsum untreated and PG,untreated the purified PG, the PG purified can be used PG can as the be retarderused as the in cementretarder instead in cement of the instead mineral of gypsumthe mineral [23 ].gypsum [23]. It wasIt was reported reported that that the the soluble soluble impurities impurities in in PG PG can can be be converted converted to to insoluble insoluble com- pounds by using lime or limestone as the neutralization agent [24,25]. In Potgieter’s pounds by using lime or limestone as the neutralization agent [24,25].◦ In Potgieter’s study [[26],26], the PG was treated by washing in tap water and dried at 50 °C.C. Subsequently, the dried dried samples samples were were soaked soaked in in a amixture mixture so solutionlution of ofthe the milk milk of lime, of lime, ammonium ammonium hy- hydroxide, hydrochloric acid, and sulphuric acid for 30 min. Finally, the treated gypsum droxide, hydrochloric acid, and sulphuric acid for 30 min. Finally, the treated gypsum was filtered and washed twice by tap water and dried at 45 ◦C. The results indicated was filtered and washed twice by tap water and dried at 45 °C. The results indicated that that milling the PG with a small amount of hydrated lime was beneficial to the strength milling the PG with a small amount of hydrated lime was beneficial to the strength de- development of the final cement, while washing PG with the milk of lime solution reduced velopment of the final cement, while washing PG with the milk of lime solution reduced the setting time and harmed the strength development because setting time retardation is the setting time and harmed the strength development because setting time retardation originated from the coprecipitated crystal structure rather than the water-soluble form of is originated from the coprecipitated crystal structure rather than the water-soluble form gypsum. of gypsum. Considering some polymers have unpaired electrons, it is a feasible way to use them to immobilize the heavy metals in PG. Based on this consideration; polyethylene glycol (PEG) and polyvinyl alcohol (PVA) were used as adsorbents to stabilize heavy metals in PG [27]. In this study, two types of PEG and two types of PVA with different molecular weights were mixed with PG to evaluate the heavy metals immobilization effects. The testing results indicated that the heavy metals can be significantly bonded in PG with the addition of these polymers. The stabilization orders of the heavy metals by using PEG Crystals 2021, 11, 719 4 of 20
and PVA are: Cu (96%) > Cr (94%) = Mo (94%) > Zn (74%) > Cd (70%) > Pb (11%), and Cu (95%) > Mo (93%) > Cr (91%) > Cd (84%) > Zn (78%) > Pb (10%), respectively. Besides, the increasing molecular weight and dosage of these polymers increased surface area and thus enlarge the electron exchange probability. Consequently, the heavy metals stabilization degree can be enhanced accordingly. Synergistic using various solid wastes is an effective approach to mitigate the negative environmental impact due to the leaching of the impurities. It was reported that, by combin- 3− ing with the electrolytic manganese residue (EMR), the soluble phosphate (PO4 ), fluorine (F−) in PG and the manganese (Mn2+), ammonia nitrogen (NH4+-N), and other heavy metal ions in EMR can be stabilized through aligning the water/binder ratio, EMR/PG ratio, pH value, temperature, and ions’ concentration of this synergistic solid waste system. The results indicated that the immobilization effect can be optimized with EMR/PG ratio of 1:2 and a pH value of 9.0. After the solidification process, the manganese ions have been solidified in the form of Mn3(PO4)2·7H2O and Mn(OH)2, ammonia nitrogen was immobilized as struvite, fluorine was stabilized in the form of (Mn, Ca, Mg)F2, and the phosphate was stabilized as (Mn, Ca, Mg)3(PO4)2 and (Mn, Ca, Mg)HPO4. The key factor of this method is using MgO to adjust the pH value of this system. The MgO will react 4+ 3− with water to form Mg(OH)2, and then with the NH -N and soluble PO4 to form the struvite. One thing that has to be noted here is that, if the pH value is higher than 10, the struvite cannot form and the generated ammonia will become the source of the secondary pollution [28]. Combining with waste fluid catalytic cracking catalyst, which is mainly the synthetic zeolite Y, the properties of PG can be significantly enhanced by using ultrasound treatment. It was found that the mechanical properties have increased about four times comparing with the control samples after the addition of the waste fluid catalytic cracking catalyst (zeolite Y) with dosage varies from 0.5% to 10% [29]. Additionally, it was found that the zeolite can be used as the adsorption additive to deal with the acidic solvable P2O5 and F contaminants in PG, which is beneficial to the setting time and hydration of the PG. The compressive strength of the samples with 5% zeolite and 2 min sonication increased 35% comparing with the control sample [30]. Similar results were obtained by Fornes [31]. It was also stated that the waste zeolite can be used as an effective adsorbent to enhance the hydration degree, reduce the soluble phosphate content 3 to 5 times and thus enhance the properties of PG. Most recently, a novel press-formed process was developed to purify the PG by using waste zeolite as an effective adsorbent of the acidic impurities [32]. This method opens an eye to realize the PG purification without introducing secondary pollution and avoiding the cost enhancement. In this method, 2.5% to 10% of waste zeolite were dry mixed with the PG and followed by press-forming under 15 MPa to 20 MPa in a cylindrical mold. The water content is about 16% which is much lower than the normal 40–60% PG preparation. After curing for 7 days with 100% humidity at room temperature, the testing results showed that although the mechanical properties have slightly reduced with a small amount of waste zeolite, the water-soluble P2O5 has significantly decreased by 3–7.5 times because the harmful P2O5 was absorbed by the porous zeolite. It is a cost-effective and environmentally friendly method to mitigate the environmental problems caused by the PG purification process.
2.2. Physical Methods Water washing is a cost-effective and facile approach to remove the impurities in PG. After washed by deionized water, part of the soluble impurities on the surface of PG crystals could be removed, and the microstructure and sizes of the PG have changed accordingly as well. The workability of the water-washed PG mixture was better than the unwashed one which benefits the transport property enhancement [33]. Wet sieving is another useful method to eliminate the impurities of PG. The PG sample was wet sieved via a 300-micron sieve, and the contents passed through and remained Crystals 2021, 11, 719 5 of 20
in the sieve were washed and dried at 42 ◦C followed by microstructure and chemical composition analysis. The results indicated that 85% of the PG has passed through the sieve, and the impurities of this fine part were significantly lower than another part that was remained in the sieve. Besides, it was found that the hydro-cyclone process is also effective to eliminate impurities [34].
2.3. Calcination Methods Except for the chemical and physical methods, another important purification ap- proach is calcination. After calcination, PG will be converted to anhydrite and the P and F impurities will become inert. It has been widely accepted that the reaction between the anhydrite and water is very slow and thus needs to be activated by alkali/alkaline sulfates, carbonates, Ca(OH)2, or a small amount of Portland cement. Singh investigated the gypsum/slag system by calcination PG at 750 ◦C for 4 h to obtain the gypsum anhy- drite. By using analytical grade Ca(OH)2, Na2SO4·10H2O, and FeSO4·7H2O as activators, the anhydrite will react with the Al2O3 in the slag to form ettringite in the presence of Ca(OH)2. Meanwhile, the SiO2 in the slag will react with the Ca(OH)2 to form tobermorite compounds [35]. The calcination temperature is an important factor that determines the purification effect of PG. By comparing the calcination temperatures varied from 200, 400, 600, and 800 ◦C with a heating rate of 10 ◦C/min, the compressive strength of the PG/slag system increased with increasing calcination temperature of PG due to the decreasing P and F impurities content [36]. In many cases, the calcination process was implemented along with chemical methods. It was found, after lime treatment followed by 500 ◦C calcination, the PG can be converted to gypsum anhydrite and thus used as the additive in cement. To accelerate the hydration process of the anhydrite gypsum, K2SO4 and hemihydrate gypsum were used as the activa- tor. The setting time, strength development, and even water-resistance were all evaluated and the results indicate a promising application potential of this method [37]. Another study claimed that the PG can be neutralized by calcinating at 800 and 900 ◦C, provided with lime washing and alkali sulfate activation [38]. Most recently, the mechanisms of the calcination modification of PG were presented. It was believed that the PG will dehy- drate to form ®-hemihydrate and III-anhydrite under low-temperature calcination, while form anhydrite with different solubility and activity under high-temperature calcination. The soluble phosphorus impurities will be converted into water-insoluble and harmless ◦ CaP2O7. Once the calcination temperature is higher than 800 C, the soluble phosphorus content will be non-detectable [39].
3. Building Materials Although most of the soluble impurities in PG could be partially removed by a few purification treatment methods, these processes will result in secondary pollution and increasing cost. As a result, it is necessary to explore new methods that can utilize the impurities rather than remove them. The following methods representing a few typical applications to recycle the PG without removing the impurities.
3.1. Raw Material for Cement Production It has been well known that the PG is hardly being directly used in the Portland cement as the retarder; however, it was found that the PG can be used as the raw material to manufacture the belite-calcium sulfoaluminate (BCSA) clinker [40]. It was found that the P and F impurities in PG are beneficial to form BCSA cement clinker above 1100 ◦C, which is about 50 ◦C lower than the clinker prepared with natural gypsum. The P and F impurities can be immobilized in C4A3Sˆ and stabilize the cubic C4A3Sˆ phase. A similar study claimed that the PG can be used as the raw material to manufacture the calcium sulfoaluminate (CSA) cement. It was found that the optimal calcination temperature is in the range of 1250 to 1300 ◦C which coincides with the decomposition Crystals 2021, 11, x FOR PEER REVIEW 6 of 20
3.1. Raw Material for Cement Production It has been well known that the PG is hardly being directly used in the Portland cement as the retarder; however, it was found that the PG can be used as the raw mate- rial to manufacture the belite-calcium sulfoaluminate (BCSA) clinker [40]. It was found that the P and F impurities in PG are beneficial to form BCSA cement clinker above 1100 °C, which is about 50 °C lower than the clinker prepared with natural gypsum. The P and F impurities can be immobilized in C4A3Ŝ and stabilize the cubic C4A3Ŝ phase. A similar study claimed that the PG can be used as the raw material to manufacture Crystals 2021, 11, 719 6 of 20 the calcium sulfoaluminate (CSA) cement. It was found that the optimal calcination temperature is in the range of 1250 to 1300 °C which coincides with the decomposition temperature of the PG, and the PG content can be varied from 7% to 27%. Besides, the decompositiontemperature ofdegree the PG, of PG and is thegoverned PG content by the can calcination be varied temperature from 7% to and 27%. the Besides, mixture the proportion.decomposition The highest degree decomposing of PG is governed content by of the PG calcination can reach temperature 36% at 1300 and°C. The the mixturehigh- estproportion. 28 d compressive The highest strength decomposing was 67 MPa content with of a PGmixture can reach proportion 36% at 1300of 59%◦C. limestone, The highest 31%28 bauxite, d compressive 10% PG, strength and calcined was 67 at MPa 1250 with °C. a The mixture impurities proportion of PG of will 59% retard limestone, the set- 31% tingbauxite, time of 10% CSA, PG, and and decrease calcined the at 1250mechan◦C.ical The properties impurities of of the PG CSA will retardcement the [41]. setting time of CSA,It has and been decrease reported the that mechanical the main properties hydration of theproduct CSA cement of ye’elimite [41]. is ettringite which contributesIt has been reportedto the rapid that theearly main age hydrationstrength development, product of ye’elimite while the is ettringite main hydra- which tioncontributes product of to ternesite the rapid is early monosulfate age strength which development, accounts for while the the late main age hydrationstrength devel- product opment.of ternesite To further is monosulfate flourish whichthe manufacturin accounts forg the techniques late age strength of CSA development. cement by using To further PG as flourishraw material, the manufacturing the belite–calcium techniques sulfoaluminate–ternesite of CSA cement by using system PG was as raw developed material, to the enhancebelite–calcium the long-term sulfoaluminate–ternesite performance of CSA system cement. was With developed a mixture to enhancecomposition the long-term of 60.6% limestone,performance 15.7% of bauxite, CSA cement. and 23.7% With a PG, mixture and a composition secondary heat of 60.6% treatment limestone, method 15.7% (shown bauxite, as andFigure 23.7% 2), PG,the belite-calcium and a secondary sulfoaluminate-ternesite heat treatment method cement (shown material as Figure was2), prepared the belite- andcalcium the early sulfoaluminate-ternesite age and late age properties cement were material tested. was The prepared results and indicated the early that age the and sec- late ondaryage properties heat treatment were tested. method The will results lead indicatedto a remarkable that the increase secondary of ternesite heat treatment content method and thuswill benefit lead to the aremarkable late age strength increase of ofthe ternesite belite–CSA content cement and [42]. thus benefit the late age strength of the belite–CSA cement [42].
FigureFigure 2. Calcination 2. Calcination process process of belite–calcium of belite–calcium sulfoaluminate-ternesite sulfoaluminate-ternesite system. system. (a) one-stage (a) one-stage cal- cinationcalcination process, process, and ( andb) secondary (b) secondary heat treatment heat treatment process process [42]. [42].
ApartApart from from the the calcination calcination process, process, PG cancan also also be be used used as as raw raw material material to manufacture to manu- facturecementitious cementitious material material without without high-temperature high-temperature treatment. treatment. By using By fly using ash, hydratedfly ash, hy- lime, dratedand PG/calcinedlime, and PG/calcined PG as raw PG materials, as raw materi a low carbonals, a low cementitious carbon cementitious binder was binder prepared. was In this system, the lime content was fixed as 10%, and the contents of fly ash and PG were prepared. In this system, the lime content was fixed as 10%, and the contents of fly ash balanced from 90% to 40% and 0% to 50%, respectively, and the PG was heated at 150 ◦C and PG were balanced from 90% to 40% and 0% to 50%, respectively, and the PG was for 2 h to obtain the calcined PG. The testing results demonstrated that the mechanical heated at 150 °C for 2 h to obtain the calcined PG. The testing results demonstrated that properties were considerably reduced with increasing PG content. However, if using the mechanical properties were considerably reduced with increasing PG content. calcined PG, the mechanical properties can be distinctively enhanced. Furthermore, the However, if using calcined PG, the mechanical properties can be distinctively enhanced. durability of this material is sensitive to the curing condition, and the volume stability is Furthermore, the durability of this material is sensitive to the curing condition, and the quite challenging when the samples are exposed to water because of the sulfate attack from volume stability is quite challenging when the samples are exposed to water because of the PG sludge [43]. the sulfate attack from the PG sludge [43]. 3.2. Component in Cementitious Materials The natural gypsum is an important component in Portland cement, the utilization of PG to replace the natural gypsum is still challenging because of the impurities of soluble P and F. It was reported that using the PG as the retarder of Portland cement rather than the natural gypsum will prolong the setting time and significantly reduce the mechanical properties of Portland cement [40]. Although it is hard to use PG directly in the Portland cement as the retarder, it was found that the PG can be used as the additive in the belite–calcium sulfoaluminate (BCSA) clinker [40]. By adding PG, the soluble P can prolong the setting time of BCSA cement and retard the hydration process of the C4A3Sˆ phase. However, similar to the Portland cement, the presence of the soluble P harms the mechanical properties development of the BCSA cement. Crystals 2021, 11, 719 7 of 20
In addition to acting as the retarder of the Portland cement, PG can also be used as a sulfate resource in the non-sintering or alkali-activated cementitious materials system [44]. It was found that the reaction between the PG and the slag is a continuous process, which is beneficial to the late age strength development of the non-sintering cement system. The microstructure analysis demonstrates that the glassy phase of the slag will be destroyed due to the alkali and sulfate activation. The PG will react with the ions from inside of the slag and form ettringite, while the remaining slag will form the C-S-H phase. In this case, the PG was not only acting as an activator but also as a binder phase accounting for the enhancement of mechanical properties. Besides, it was reported that the pH value is a critical factor that determines the formation of ettringite. If the pH value was higher than 13, the acid film of the slag would be broken and the thus confining the formation of the ettringite. Another study reported, if mixing 45% PG, 10% steel slag, 35% ground granular blast furnace slag, and 10% of limestone, the compressive strength of the samples can be as high as 40 MPa. As in Mun’s statement, it was also believed that the slags are activated by alkali and sulfate, and the hydration products are C-S-H phase and ettringite [45]. Similarly, a ground granulated blast furnace slag system combining with PG was investigated to manufacture the super-sulfate cement [46]. It was found that the PG was involved in the solid reaction process forming portlandite and ettringite instantly after dissolution. Unlike the CSA/PG system, in which the addition of PG will considerably reduce the compressive strength of the system, if using sodium hydroxide as the activator, the compressive strength of the GGBFS/PG system can be considerably enhanced due to the sodium hydroxide will increase the content of amorphous binder phase, and the alkali activation degree of the sodium hydroxide is stronger than the sodium silicate. The microstructure analysis results indicated that the amorphous C-A-S-H gel structure was the hydration product and the addition of PG will decrease the Al/Si and Ca/Si ratios which is beneficial to form polymerized network structure of the super sulfate cement system. In the slag/PG system, the PG content can be in a wide range from 10% to 50%. Provided with an appropriate activation method, the compressive strength of the slag/PG system can reach as high as 45 MPa with a composition of 45% PG, 48% slag, 7% cement clinker, and 1% activator [47]. Microstructure and phase structure analysis indicated that the ettringite, portlandite, and the dihydrate gypsum crystals were interacted with each other and formed a compact structure, which was embedded in amorphous C-S-H gels. PG can also be used as a major component of oil well cement through a step-by-step method [48]. In this study, the PG content was fixed with 50%, and the oil well cement, ground granular blast furnace slag, activators, silica fume, retarder, and fluid loss control additive were optimized in a proper mixture design according to step-by-step properties testing results. It was found that the optimal weight ratio of PG/slag/oil well cement was 5/3/2, and the 2 d compressive and flexural strengths reached 12.9 and 8.3, and 18.5 and 10.6 MPa, at 50 ◦C and 80 ◦C, respectively. Comparing with the control oil well cement, the linear expansion rate of the PGS sample increased by 874% at 50 ◦C, and 169% at 80 ◦C. This performance is beneficial to mitigate the shrinkage compensation of the oil well cement. The microstructure analysis results demonstrate that the hydration products of the PGS are mainly the ettringite, gypsum, and C-S-H gel. With the addition of PG, the pore volume and permeability are much lower than the pure oil well cement material.
3.3. Cemented Paste Backfill Another important application of PG is utilizing it as cemented paste backfill. In this application, the mixing time is a critical parameter that governs the final properties of the backfill materials. Seven mixing times, ranged from 5 to 240 min, were set as the variables to evaluate and analyze the setting time, workability, density, porosity, microstructure, and mechanical properties. It was found that 60 min is the optimized mixing time. In this period, the apparent viscosity of the backfill paste sharply decreased due to the particles in the paste prone to parallel lining up in the shear direction and lead to deflocculation. With the mixing time extended to 240 min, the apparent viscosity gradually increased because of Crystals 2021, 11, 719 8 of 20
the cohesion of the hydration products. The mechanical properties testing results indicated that the compressive strength is proportional to the mixing time in the range of 5 to 60 min, while in the range of 60 to 240 min, the mechanical properties declined with the extension of mixing time due to the break of the hydration production skeleton [49]. Synergistic using PG and other waste materials is a promising approach to enhance the properties of the cemented paste backfill. Using PG, phosphate tailing (PT), cement, and CaO as raw materials, the cemented paste backfill was prepared and the properties were optimized by comparing the PG/PT ratio of 0/1, 1/3, 2/2, 3/1, and 1/0. To further enhance the performance, slag was added in the PG/PT system with ratios of 0.2, 0.4, 0.6, and 0.8 by fixing the PG/PT ratio of 1/3. It was found that the compressive strength of the PG/PT system is too low to be applied as cemented paste backfill. Although with the addition of cement, the compressive strength of the system has remarkably enhanced, the emitted H2S gas confined its potential application as backfill. To solve this problem, 2% CaO and 4% cement were mixed with the PG/PT system, and the gas emission problem has been solved and the compressive strength has been significantly improved [50]. Apart from the phosphorous tailings, the construction demolition waste (CDW) and PG were mixed with cement to manufacture the cemented paste backfill. The results revealed that the solid concentration of cemented paste backfill increased from 60% to 70% and the setting time decreased from 20% to 30%, provided with the addition of the construction demolition waste in a range of 0% to 40%. The highest compressive strength reached 1.74 MPa with 40% CDW, 1:6 cement/water ratio, and 70% of solid concentration. The most important achievement of this study is that the environmental indices from the bleed water and leachates demonstrated that this material can satisfy the category III requirements of Chinese environmental standard (DZ/T 0290-2015) [51]. As mentioned, the environmental impact, Li and Shi’s group investigated the immobi- lization of PG as the cemented paste backfill material. In this study, the cemented paste backfill material was prepared by mixing the PG and cementitious agents with a ratio of 4:1 and a slurry concentration of 66%. After cured for 120 days, the release of P, F, and heavy metals was significantly decreased, and after five round of leaching tests, the risk of pollutants diffusion was much lower than the leachates from the PG directly, and the heavy metals contents in the leachates were satisfied with the Chinese standard (DZ/T 0290-2015) [52].
3.4. Additives in Pavement Materials Since the bitumen was considered an environmentally friendly binder material to stabilize the naturally occurring radioactive materials [53], using waste PG as an additive in pavement engineering has become a win-win approach to eliminate the negative envi- ronmental problems of recycling. It was demonstrated that PG can serve as the modifier of bitumen along with a small amount of sulfuric acid [54,55]. The results indicated that sulfuric acid is beneficial to release the phosphorus in the crystalline structure of PG, and thus enhance the rheological properties of the bitumen. Another study claimed that the PG can be used as a foaming agent to prepare foamed bitumen materials due to the molecular water release from the PG [56]. This is a novel technology by using the molecular water in PG to realize the internal foaming rather than the addition of external water. Based on this designation, the warm mix asphalt can be realized due to the water foam can significantly reduce the mixing temperature of bitumen and thus reduce greenhouse gas emissions, decline the energy consumption, and improve the working condition of the operators. The testing results indicated that the addition of PG (also along with 0.5 wt.% of sulfuric acid) as a modifier can reduce the mixing temperature of bitumen from 180 ◦C to 150 ◦C, and the radioactivity and metal concentration both satisfy the European standards. Crystals 2021, 11, 719 9 of 20
4. Useful Chemicals 4.1. Fertilizer Waste PG can be used as raw material to manufacture fertilizers. By using sulfuric acid to remove the impurities, a highly purified ammonium sulfate fertilizer was prepared by using PG as raw material. In this manufacturing process, first, 1 kg of grounded PG powder (200 mesh size) was soaked in 4L sulfuric acid with a concentration of 8M and stirred with 150 rpm for 30 min at 80 ◦C. Second, 500 g of the obtained precipitates were soaked in 2 L of ammonium sulfate with a concentration of 3%, solid to liquid ratio of 0.25, and pH value of 7. Third, the slurry was reacted with 10% excess amount of ammonium carbonate higher than the stoichiometric ratio with 150 rpm for 3 h. Finally, 250 mg barium chloride was added to the solution and hold at 55 ◦C for 4 h to remove the radium. The obtained ammonium sulfate can reach a purity of 95% [57].
4.2. Rare Earth Elements Recovery In a Polish case study, it was found that the Lanthanides rare earth elements (REE) can be extracted from the local PG via a facile chemical method. In this extraction process, 20 g PG was soluted in sulfuric acid solutions with concentrations of 10%, 15%, and 20% and a liquid-to-solid ratio of 1:3 and 1:4 at room temperature. The obtained suspension was stirred for 1h and subsequently separated in a centrifuge with the speed of 6000 rpm for 5 min. The separated solution was used to extract the lanthanides REE by using precipitation and concentration methods. In this laboratory precipitation process, the oxalic acid solution was used to precipitate the lanthanide REE oxalates. The obtained oxalates were filtered, dried, and thermally decomposed to produce the lanthanide oxide concentrates, which were used as the raw material to separate the individual elements. In addition to the concentration method, alternative alkaline treatment of the solution can be used as the precipitation of lanthanide hydroxide precipitation. In this method, NaOH, Na2CO3, and even KOH can be used to solve the lanthanide REE, and the remained potassium solution can be used to produce mineral fertilizers [58]. Similar methods were used to extract the REE from PG by aligning the processing parameters, including the grinding treatment, ultrasonic impact, and resin-in-pulp process, as shown in Figure3[ 59]. Apart from using the sulfuric acid, a mixed acid solution, including hydrofluoric acid, nitric acid, and perchloric acid with a ratio of 6:5:3, was used to extract the REE from PG. The results reveal that the crystallized solid after the evaporation process can reach as high as about 86% [60]. However, a most recent life cycle assessment study claimed that current developed technology is cost-effective but needs high financial investment and has a high potential risk degree. As a result, new technology, such as membrane technology or other renewable energy sources should be considered as promising methods [61]. Crystals 2021, 11, 719 10 of 20 Crystals 2021, 11, x FOR PEER REVIEW 10 of 20
FigureFigure 3.3. FlowFlow chartchart ofof REEREE recoveryrecovery fromfrom PGPG viavia leachingleaching byby sulfuricsulfuric acidacid [[59].59].
4.3.4.3. NanocrystallineNanocrystalline MaterialsMaterials NanocrystallineNanocrystalline hydroxyapatitehydroxyapatite can bebe manufacturedmanufactured byby usingusing PGPG andand potassiumpotassium dihydrogendihydrogen inin aa hydrothermalhydrothermal condition.condition. TheThe processingprocessing parametersparameters werewere temperature,temperature, reactionreaction time, and pH pH value value of of the the solution, solution, and and the the purity purity of ofthe the raw raw materials. materials. The The op- ◦ optimizedtimized synthesis synthesis condition condition was was soaking soaking the mixture atat200 200 C°C for for 15 15 h withh with a pHa pH value value of 11.of 11. SEM SEM and and TEM TEM images images demonstrated demonstrated that the that synthesized the synthesized hydroxyapatite hydroxyapatite nanocrystals nano- arecrystals rod shapeare rod with shape a various with a various aspect ratioaspect which ratio waswhich governed was governed by the by concentration the concentra- of thetion Brij-93 of the surfactant.Brij-93 surfactant. The Brij-93 The surfactantBrij-93 surf willactant wrap will the wrap surface the ofsurface the hydroxyapatite of the hydrox- crystals,yapatite andcrystals, thus resultand thus in a result preferential in a preferential growth in the growth longitudinal in the longitudinal direction and direction prevent theand agglomeration prevent the agglomeration of these nanocrystals. of these Figurenanocrystals.4 gives the Figure TEM 4 image gives ofthe the TEM synthesized image of hydroxyapatitethe synthesized withhydroxyapatite various concentrations with various of concentrations Brij-93 surfactant of Brij-93 [62]. Anothersurfactant study [62]. demonstratedAnother study thatdemonstrated PG can react that with PG can phosphoric react with acid phosphoric in an alkaline acid in environment an alkaline en- by aligning the pH value with ammonia solution. The residue was dried at 80 ◦C and followed vironment by aligning the pH◦ value with ammonia solution. The residue was dried at by80 calcination°C and followed at 600 and by 800 calcinationC for 2 h to at obtain 600 theand nano-hydroxyapatite 800 °C for 2h to crystals. obtain TEM the morphologynano-hydroxyapatite (Figure5 )crystals. showed TEM the rod morpholo shape particlesgy (Figure size 5) ofshowed 50 nm inthe length rod shape and 5parti- nm in diameter. For the samples calcined at 900 ◦C, the obtained particles are spherical cles size of 50 nm in length and 5 nm in diameter. For the samples calcined at 900 °C, the with diameters in the range of 54–74 nm [63]. Similar work stated that a mixture of obtained particles are spherical with diameters in the range of 54–74 nm [63]. Similar calcium hydroxyapatite and tricalcium phosphate could be obtained by thermal treatment work stated that a mixture of calcium hydroxyapatite and tricalcium phosphate could be methods [64]. obtained by thermal treatment methods [64]. High-quality nanocrystalline calcium fluoride (CaF2) has been prepared via the precip- High-quality nanocrystalline calcium fluoride (CaF2) has been prepared via the pre- itation method by using calcined PG as raw material. Before preparing, PG powders were cipitation method by using calcined PG as raw material. Before preparing, PG powders pre-washed with dilute alkali and acid solutions, and distilled water to remove soluble were pre-washed with dilute alkali and acid solutions, and distilled water to remove impurities. The purified PG powder was then mixed with sulfur and activated carbon with soluble2 −impurities. The purified PG powder was◦ then mixed with sulfur and activated the SO4 /S molar ratio of 0.1, and calcined at 800 C for 120 min in a nitrogen atmosphere. carbon with the SO42−/S molar ratio of 0.1, and calcined at 800 °C for 120 min in a nitro- The residues were dissolved with HCl. Then, the mixture was filtered and NH4F was addedgen atmosphere. with various The Ca/F residues molar were ratios. dissolved After the with reaction, HCl. Then, the suspension the mixture was was separated filtered byand centrifuge, NH4F was and added the residuewith various was washed Ca/F molar with HCl,ratios. distilled After the water, reaction, and dried. the suspension The XRD andwas SEMseparated results by indicated centrifuge, that and the the average residue particle was washed diameter with was HCl, around distilled 70 nm water, and and the crystallinedried. The of XRD the product and SEM was results 69.36% indicated [65]. that the average particle diameter was around 70 nm and the crystalline of the product was 69.36% [65].
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FigureFigureFigure 4. TEM 4.4.TEM TEM images imagesimages of ofthe of the thesynthesized synthesized synthesized hydroxyapatite hydroxyapatite hydroxyapatite nanocrystals nanocrystals nanocrystals with with with various various various concentration concentration concentration of of Brij-93Brij-93of Brij-93 surfactant surfactant surfactant ( (aa)) no no(a) surfactant, surfactant,no surfactant, ( (bb)) 0.003,( 0.003,b) 0.003, ( (cc)) 0.006, 0.006,(c) 0.006, and and and ( (dd)) 0.01( 0.01d) 0.01 mol mol mol [62]. [62 ].[62].
FigureFigureFigure 5. TEM 5.5.TEM TEM images images images of ofthe of the thesynthesized synthesized synthesized hydroxyapatite hydroxyapatite hydroxyapatite nanocrystals nanocrystals nanocrystals ( (aa)) before(a) before calcination,calcination, calcination, (b(b)) cal- (b) calcinedcinedcalcined at at 600 600 at 600°C,◦C, and°C, and and ( (cc)) calcined calcined(c) calcined at at 900 900 at 900°C◦C[ [63]. °C63 ].[63].
NanocrystallineNanocrystalline calcium calcium carbonate carbonate can can be manufacturedbe manufactured from from PG PGvia viaa crystalliza- a crystalliza- 2 3• 2 tiontion process process in ain gas a gas(CO (CO2)-liquid(NH)-liquid(NH3•H2O)-solidH O)-solid (PG) (PG) three-phase three-phase system. system. PG PGwas was
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Nanocrystalline calcium carbonate can be manufactured from PG via a crystallization process in a gas (CO2)-liquid(NH3•H2O)-solid (PG) three-phase system. PG was mixed with deionized water with a weight ratio of 1:9 to 1:7. Subsequently, the obtained slurry 2− was mixed with ammonium hydroxide with ammonia to SO4 the molar ratio of 2.3 to 2.5. After reached the setting temperature, CO2 with a fixed flow rate was pumped into the slurry with continuous stirring. After filtration, the concentration of sulfate ions was quantified with the help of BaCrO4 solution. When the pH value was lower than 8, the reaction was terminated, and the slurry was filtered and dried to obtain the CaCO3 powder. The critical factors, including the temperature, gas flow rate, and reaction time, which determine the particle sizes of the nanoparticles have been investigated. The results demonstrated that with the CO2 flow rate of 251–138 mL/min, the average particle sizes of the nanoparticles were in a range of 86–104 nm at the temperature range of 30–40 ◦C. The reduction of reaction time and the amount of soluble impurity in PG were beneficial to the formation of the CaCO3 nanoparticles [66].
5. Sustainable Composites A PG and fly ash composite was developed to improve the mechanical properties of the system. In this system, the pozzolanic reaction of the fly ash was activated by sulfate. After calcined at 135 ◦C, the milled PG was added to the fly ash-lime composite with the dosage range of 0–15%. The mechanical properties testing results revealed that the compressive strength of the samples improved with increasing PG content. The potential mechanisms of the reinforcement are originated from serval aspects. First, PG dehydrates will react with alumina in fly ash and calcium oxide in lime to form ettringite which helps to accelerate the pozzolanic reaction of the fly ash. Second, the reaction between the lime and the impurities in PG form a series of stable compounds that mitigate the negative effects of the impurities on the mechanical properties development of the fly ash-lime composite system [19]. A load-bearing wall brick composite was manufactured by using PG, fly ash, lime, and sand as raw materials. The PG was pretreated by steaming and followed by an autoclave process. PG types, contents, and vapor pressure were used as the parameters to optimize the final properties of the composites. The results indicated that the steam treatment has a distinctive effect on the mechanical properties of the composite due to the recrystallization of the PG. The strength of the PG-fly ash-lime-sand composite reduced with increasing PG content and improved with increasing lime content. The same as the previous study, it was also claimed that the presence of lime is beneficial to stabilize the impurities of the PG and help the calcium silicate reaction. After 15 freeze/thaw cycles in the temperature range of −20 ◦C to 20 ◦C, the remained compressive strength was 18.6 MPa and the weight loss was only 0.029%, suggesting good durability of the composite system [67]. A similar study stated that this PG/fly ash/lime composite has satisfactory performance for geotechnical applications [68]. A non-fired brick composite was prepared from the waste PG. In this manufacturing process, the pre-pressed green bricks were hot dried at 180 ◦C to convert the dihydrate gyp- sum to hemihydrate gypsum and followed by water-soaked to form in situ recrystallized gypsum. After air dried naturally, the non-fired brick products were obtained. The results showed that the optimized mixture proportion was 75% PG, 19.5% river sand, and 4.0% Portland cement, and 1.5% hydrated lime. The compressive strength, water-saturated com- pressive strength, and flexural strength are 21.8 MPa, 13.7 MPa, and 5.2 MPa, respectively. The microstructure analysis demonstrated that the in situ recrystallized gypsum crystals are regular, dense, and tangled with each other to form a stable interlock structure [69,70]. To further improve the mechanical properties of the non-fired composites, the calcium sulfate whisker was used as the modifier to reinforce the non-fired composite system. It was found that by adding 1 wt.% of calcium sulfate whisker, the bending strength of the non-fired composite reached 27.2 MPa, enhanced over 80% compared with the control sample [71]. Crystals 2021, 11, 719 13 of 20
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In addition to the calcium sulfate whisker, mineral, glass, and polypropene (PP) fibers were used to reinforce the PG/slag composites. The impact resistance, flexural strength, strength,water, and water, freeze/thaw and freeze/thaw resistance resistance of these fiber-reinforcedof these fiber-reinforced composites composites were tested. were In tested.these threeIn these types three of fibers, types theof fibers, PP fiber the shows PP fiber a good shows dispersion a good conditiondispersion and condition has the and best hasbonding the best strength bonding with strength the matrix. with 1.35 the vol.% matrix. of PP1.35 fiber vol.% increased of PP fiber the flexural increased strength the flex- two uraltimes strength and impact two times resistance and impact 7 times resistance [72]. 7 times [72]. ToTo decrease decrease the the manufacturing manufacturing cost, cost, most most recently, recently, a a high-strength high-strength PG-based PG-based paper paper andand fiber-free fiber-free plasterboard plasterboard have have been been developed developed by by using using a a facile facile intermittent intermittent pressing pressing hydrationhydration method. method. This This method method is based is based on pressing on pressing the dihydrate the dihydrate gypsum gypsum powder powder into ainto green a green plasterboard plasterboard green green body body and and followed followed by byimmersed immersed in inwater water to to convert convert the the hemihydratehemihydrate gypsum gypsum into into dihydrate dihydrate gypsum, gypsum, in in which which the the crystals crystals are are interlocked interlocked with with eacheach other. other. As As the the green green body body was was repeatedly repeatedly pressed pressed so so that that the the swelling swelling of of the the green green bodybody results results in furtherfurther deepdeep hydration hydration till till the the fully fully dense dense of theof composite.the composite. Figure Figure6 gives 6 givesthe flowchart the flowchart of the of manufacturing the manufacturing process. process. The The optimized optimized humidity humidity was was 20%, 20%, and and the thePG-based PG-based green green body body was was pressed pressed 3 times 3 times with with 10 min 10 min intervals intervals and and a value a value of 20 of MPa. 20 MPa.The finalThe bendingfinal bending strength strength can reach can reach 14.7 MPa14.7 [MPa73]. [73].
FigureFigure 6. 6. Flowchart ofof thethe intermittent intermittent pressing pressing hydration hydration process process for preparingfor preparing paper paper and fiber-free and fi- ber-freeplasterboards plasterboards [73]. [73].
AA recent studystudy investigatedinvestigated the the feasibility feasibility of usingof using PG toPG prepare to prepare porous porous sound- absorbing composite. The β-hemihydrate PG (β-HPG) was obtained by low-temperature sound-absorbing ◦ composite. The β-hemihydrate PG (β-HPG) was obtained by low-temperaturecalcination (160 C)calcination of PG. The (160 expanded °C) of perlite,PG. The basalt expanded fiber, pore-forming perlite, basalt agent, fiber, and pore-formingretarder (Protein agent, retarder, and retarder SC) were (Protein used asreta rawrder, materials. SC) were The used dosage as raw of materials. the form agent The dosagewas in of a rangethe form of 0–0.6 agent wt.% was of in the a range HPG, of basalt 0–0.6 fiber wt.% content of the wasHPG, in abasalt range fiber of 0–2 content wt.%, the expanded perlite content was in a range of 0–8 wt.%, and the SC retarder was fixed to was in a range of 0–2 wt.%, the expanded perlite content was in a range of 0–8 wt.%, and 0.2 wt.%. The PG, expanded perlite, and a pore-forming agent was mixed first for 30 s, and the SC retarder was fixed to 0.2 wt.%. The PG, expanded perlite, and a pore-forming the basalt fiber was sonicated for 15 min and mixed with binder mixture for 2 min. The agent was mixed first for 30 s, and the basalt fiber was sonicated for 15 min and mixed microstructure analysis indicated that the micropores were mostly interconnected among with binder mixture for 2 min. The microstructure analysis indicated that the micropores the hydration products, leading to the increase of open pores and sound-absorbing property. were mostly interconnected among the hydration products, leading to the increase of While the porosity is higher than a critical value, the sound absorption property will be open pores and sound-absorbing property. While the porosity is higher than a critical declined. The optimized pore-forming agent content is 0.6 wt.% of the β-hemihydrate PG. value, the sound absorption property will be declined. The optimized pore-forming Basalt fiber and expanded perlite are both beneficial to the sound absorption property of the agent content is 0.6 wt.% of the β-hemihydrate PG. Basalt fiber and expanded perlite are composite. The basalt fiber can increase the open porosity and form more interconnected both beneficial to the sound absorption property of the composite. The basalt fiber can increase the open porosity and form more interconnected pores to further improve the sound absorption property. The optimized content of basalt fiber and expanded perlite are 1 wt.% and 6 wt.%, respectively [74].
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pores to further improve the sound absorption property. The optimized content of basalt fiber and expanded perlite are 1 wt.% and 6 wt.%, respectively [74].
6. Environmental Impacts 6.1. Soil The sodic soils contain diverse soluble salts and exchangeable sodium which signif- icantly affect the growth of rice-wheat cropping. Surface crusting and water infiltration limitations are the critical problems that need to be solved to alleviate the sodic soils by using Ca2+ cation to replace the Na+ sites [75]. Using PG as the soil modifier (with a concentration of 10 Mg/hm2) can reduce the pH value of the soil and the exchangeable sodium concentration [76]. In Spain’s case, using PG as the soil modifier with a rate of 25 Mg/hm2 every 2–4 years can decrease the exchangeable sodium percent from over 15% to less than 5% [77]. However, excessive PG amount will lead to the concentration of the salt in soil and thus result in the decreasing of modification effects. As a result, it is essential to determine the PG implementation rate to optimize the modification effect. Different from the sodic soils which were mainly determined by the exchangeable sodium percentage, for acid soils, the acidity is primarily originated from the amount of the exchangeable Al3+ ions. The addition of PG can mitigate the acidity of the soil by declining the Al3+ and increase the Ca2+ content. Another study demonstrates that the application of PG mixing with lime is beneficial to neutralize the acidity of the soils and enrich the root development due to the enrichment of Ca content in the deep soils [78]. Similar to the sodic soils, the application rate of the PG is also a critical factor that determines the effect of the modification. It has been proved that the PG can immobilize heavy metals in contaminated soils [79]. A canola growth case in contaminated soils was evaluated by comparing using PG alone and PG/compost mixture as modifiers [80]. The results indicate that the canola plants enhanced over 66% comparing with the untreated soil by using PG/compost mixture as fertilizer with a concentration of 10g per kg of dry soil. However, the heavy metals immobilization effect of PG alone is greater than the PG/compost mixture because of the formation of surface metal complexes including Cd-OH, Zn-OH, and Pb-OH. In addition, the P will interact with Pb to form a pyromorphite-like mineral which mitigates the leaching of the heavy metals. Similar results were obtained by testing the heavy metal immobilization effect of PG in contaminated soils [81]. It was reported that waste PG has a great potential to reduce the heavy metal leaching in the contaminated lands. The transport of Cd, Cu, and Pb has been thoroughly changed. It was stated that this transport change was originated from a sorption mechanism of PG. The metal travel times were palpable longer in the contaminated soil with the addition of PG. The retardation values of Cd, Cu, and Pb all increased about 30%., and it was estimated that the leached mass of Cd, Cu, and Pb from the soil would be decreased by about 60–99% with the addition of PG. Another important branch of contaminations related to PG is the leaching behavior of the heavy metals from internal PG rather than the immobilization of heavy metal for the contaminated soils. Hassoune [82] summarized the leaching behavior of heavy metals present in PG. It was claimed that the transport of heavy metals can be divided into two categories, namely the As, Ba, Cd, Cr, and Ni were called the moderate mobility, and the Cu, Pb, Se, and Zn the low mobility. Furthermore, the transport of the heavy metals from the PG is a leverage process, which can absorb and release the elements during the leaching by a filtration-percolation phenomenon.
6.2. Radioactivity The argument about the radioactivity of PG never stops according to various studies. It was claimed from a Brasil study that the recycling PG can be used as building materials due to there is no additional health risk from the radioactivity perspective [83]. Similarly, a recent study from Serbia reported that the average activity concentration of 226Ra, 232Th, Crystals 2021, 11, 719 15 of 20
and 40K of PG samples was 600 ± 28 Bq kg−1, 3.2 ± 0.3 Bq kg−1, and 47 ± 16 Bq kg−1, re- spectively. The radon emanation coefficients of natural gypsum and PG were 6.73 ± 0.18% and 3.92 ± 0.04%, respectively. The estimated indoor radon concentrations for most of the samples were below the lower limit of 100 Bq m−3 recommended by the World Health Organization [84]. However, different studies claimed that before the extensive application of PG, the elimination of the radioactive elements is one of the most critical requirements. It was claimed that 85% of the radioactive 226Ra can be removed from PG via using chemical methods, including acid treatment, hybrid water treatment, and calcium carbonate powder treatment [27]. In this study, the radium equivalent, the ©index, the hindex, the absorbed ©dose rate, and the corresponding annual effective dose have been calculated to assess the radioactive hazard of PG material. The results indicated that, after treatment, the PG can be safely used as building materials and other applications from the radiation protection perspective. An Egypt study illustrated that the radioactivity of 40K, 232Th, and 238U in the PG pose a radiological threat to the creature’s health. The average radium equivalent is higher than the recommended value of 370 Bq kg−1 [85]. Another Egypt study found that the radioactive contents of various phosphate samples were significantly different. The average activity concentrations of 235U, 238U, 226Ra, 232Th, and 40K were 457, 123, and 819 Bq/kg of the phosphate fertilizers, 47, 663, 550, 79, and 870 Bq/kg of the PG and 25, 543, 409, 54 and 897 Bq/kg of the single super phosphate. It was concluded that the radiological parameters were higher than the hazard standards [86]. Apart from the soil radioactivity impact, the atmosphere impact from the stack of PG is also an environmental concern that risks the health of the living nexus of the stacking area [81]. It was stated that 90% of Po and Ra derived from the phosphate rock have remained in the waste PG, whereas the content of U is lower than 20% [87]. The potential problem of PG was the emanation of 222Rn which was originated from the a-decay 226Ra. In the United States, the USEPA (National Emission. Standards for Hazardous Air Pollutants) specifically restricts the disposal of PG must meet the standards of the National Emission Standards for Hazardous Air Pollutants (NESHAP) and the National Emission Standards for Radon Emission from PG Stacks [88]. The EPA requires that the radioactivity of PG must be lower than 370 Bq/kg of 226Ra for the usage of agricultural soil, and the flux density of 222Rn gas entering the atmosphere from the surface of a 226Ra-bearing material must be lower than 0.74 Bq/m2/s [89]. Due to the radioactivity of PG diversifies in different places, as a result, it is necessary to test the radioactivity of specific PG before it can be used in the fields. In addition to the soil and atmosphere impact, the radioactivity of PG-based building materials needs to be concerned as well. A recent study investigated the radioactivity of an ettringite-based mortar made from ladle slag and PG. The results indicated that the radon emanation decreased with increasing PG content due to the micro-porosity, and the specific surface areas were about 30 times lower than the normal Portland cement. Furthermore, the immobilization degree of contaminants was mostly higher than 90%. It claimed that the PG-based mortar has a great potential to be practically applied in many fields [90]. Another most recent study investigated the radioactivity of PG-based bricks. In this study, PG with a 226Ra content of 500 Bq/kg was used as an additive of clay with a mass ratio of 0–40%. The radiological characteristics of the prepared bricks were tested and monitored. The results indicated that the indoor radon concentration was lower than 100 Bq/m3, but the external gamma exposure should be concerned [91].
7. Concluding Remarks and Future Trends In summary, the recycling use of PG as sustainable materials is critical from both cost and environmental perspectives. However, the challenges remain severe before the wide application due to impurities, heavy metals, and radioactivity pollution. Despite the exploration of clean and cost-effective methods never stopped, the current approaches all have their inevitable short slabs. In the purification process, despite the purification effect Crystals 2021, 11, 719 16 of 20
sounds great, chemical methods lead to an increase in cost, and extra treatment processes have significantly confined the practical applications. Facile processes and cost-effective and environmentally friendly chemicals are recommended. The merits of the physical methods are easy operation and cost-effective, yet secondary pollution is a hard issue that needs to be considered. Being activated by alkali/alkaline sulfates, carbonates, Ca(OH)2, or a small amount of Portland cement, the impurities of PG can be eliminated effectively. However, the calcination method is an energy-intensive process that accounts for cost enhancement and additional carbon emissions. As a result, it is recommended to use chemicals that can reduce the calcination temperature and calcination time. The recycled PG can be used in many fields, including the retarder in cement man- ufacturing, a component in cementitious materials, cemented-based backfill, fertilizer, rear earth elements recovery, and nanocrystalline materials. In addition, waste PG can be used to manufacture bricks, high-strength boards, or porous sound-absorbing composite. By combining with other waste materials or using fiber reinforcement, the mechanical properties of the waste PG can be significantly enhanced, which encourages the recycling of the waste PG. However, before the wide applications in the above-mentioned fields, the environmental impact of the waste PG is a critical issue that must be considered, especially the heavy metal leaching and the radioactivity concerns. Despite several studies claimed that the waste PG is beneficial to immobilize the heavy metals in soil and the radioactivity is negligible, many studies argued that the potential risk from these aspects is harmful to human and creature’s health. To improve the properties and eliminate the negative environmental impacts, further research areas should be focused on the following aspects in the next steps: 1. From the purification process, more attention can be expanded to develop or find out new or appropriate chemicals that can reduce the calcination temperature and activate the inert minerals in the waste PG. 2. Recycling the disposed water after washing is necessary to mitigate the secondary pollution problem. 3. Although using waste PG to manufacture building materials is quite effective to reduce the piles’ problem, applying it to produce useful chemicals, especially nanocrystals is another important branch to recycle the waste PG. 4. Systematic investigation needs to be implemented to find out the radioactivity of the waste PG when it is used as soil fertilizer or any other product.
Author Contributions: K.R. and K.Z. prepared most of this manuscript. X.J. was in charge of the cementitious composites section, S.Z. was in charge of the environmenttal impacts section, and L.F. was in charge of the nanocomposites section and organized the figures and tables. N.X. revised the manuscript, X.C. and N.C. organized the outline and the whole contents. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Project Nos. 51772128 and 51632003), Youth Innovation Support Program of Shandong Colleges and Universities, Taishan Scholar Program, Case-by-Case Project for Top Outstanding Talents of Jinan, Double Hundred Foreign Expert Program (WST2018011), and Lianyungang Gaoxin District International Scientific Cooperation Program (HZ201902), Sino-European Research Center program (SERC-M-20191201), and Lianyungang Haiyan Plan program (2019-QD-002). Acknowledgments: In this section, you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments). Conflicts of Interest: The authors declare no conflict of interest. Crystals 2021, 11, 719 17 of 20
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